Summary Horizontal distribution with respect to vegetation patchiness, population dynamics and life cycle of Tetracanthella stachi were studied in mountain beech and spruce forests. Four vegetation patches were sampled in a 180 year old beech forest and another four in a 70 year old climax spruce forest. Both forest stands were situated approximately 250 m from one another. At each of eight plots 10 samples to the depth of 5 cm were taken monthly from May toOctober 1997 and from May to November 1998. T. stachi occurred with 100% constancy at all studied vegetation patches. Its density did not significantly differ ( P = 0.246) between the beech and the spruce forests, but it significantly differed among vegetation patches ( P< 10 —7). The species has one generation per year. The population consisted only of adult specimens in May. Reproduction took place in May, immediately after the snow cover had melted. Recruitment occurred during June and the population reached maximum density almost at all the patches. After the recruitment the number of both adults and juveniles decreased. The juveniles grew rapidly during summer and reached the body length of adults in autumn. Developed genital openings were first noticed in August and in October it was possible to sex all the juveniles. Sex ratios of the juveniles and adults did not differ significantly (α = 0.05) from 1:1 in almost all the samplings. There was a significant difference in body length between males and females (a mean body length of males 0.97—1.03 mm, of females 1.07—1.14 mm). T. stachi lives less than two years, without overlapping of adults of subsequent generations. Only few pre-adult specimens were extracted from samples taken under snow in March 2000. Pre-adults are probably inactive during winter and therefore hardly extracted in Tullgren funnels.